Metal air cells have the highest energy density in the present popular battery systems because only the anode reaction material is packaged within the cell, and the cathode reaction material is oxygen, which is drawn from the surrounding environment. Such cells are usually disk-like in appearance and are called button or coin cells. These cells are ideal power sources for small electronic devices such as hearing aid. An alternative embodiment metal air cell is the big square type for beacons or lanterns.
Metal air button cells take in atmospheric oxygen, and convert the oxygen to hydroxyl ions in the air cathode. The hydroxyl ions then migrate to the anode, where they cause the metal contained in the anode to oxidize. Usually the active anode material in such cells comprises zinc. Such zinc air button cells for hearing aids are well known, and are more fully discussed in references such as U.S. Pat. Nos. 4,041,211 (Wiacek) and 4,105,830 (Kordesch).
In recent years, the increased power consumption of the latest models of hearing aids, such as digital hearing aids, has increased demand for higher power cells. The current output of the zinc air button cells is limited mainly by two factors. First, the rate of mass transport of oxygen into the cathode limits the reaction of the oxygen with water in the alkaline environment. The present invention, however, does not address the oxygen diffusion limitation of the cathode. Second, the conductivity of the air cathode also limits the current output of the cell. In short, the air cathode is generally a poor conductor of electricity, and the present invention is directed to improve this conductivity in a novel manner.
The prior art air cathode generally comprises three layers. The active layer consists of an admixture of active carbon, catalysts, preferably manganese dioxide, and polytetrafluoroethylene (PTFE). Such activating chemicals are supported by a current collecting substrate layer. A current collector usually consists of a cross-bonded screen having nickel-metal strands woven therein, or a fine-mesh expanded metal screen. In practice, the collector is commonly merged into the active layer and surrounded by the activating chemicals of the active layer. A third layer usually consists of one or more pure PTFE membranes, which are impermeable to water and alkaline solution but are permeable to oxygen in the air. This third layer, know as an air diffusion layer, is then laminated to the active layer and the air cathode is complete.
The current collecting substrate is absolutely necessary in this kind of air cathode assembly, as the PTFE air diffusion layer is not conductive. The current generated during electrochemical reaction must pass to the outside circuit from the active layer through this metal collector. Many manufacturers are using this kind of cathode in the metal air battery industry. It is a practical solution to make the cathode more conductive. However, adding a metal screen collector into the active layer is a complex technology.
Another kind of air cathode structure, in mass production for decades in China, does not comprise a current collecting substrate in the active layer. In this kind of air cathode, the active layer is essentially the same as above but the air diffusion layer is conductive, i.e. not pure PTFE membranes. This air diffusion layer is made of polymeric materials such as PTFE and carbon black mixed together so it is conductive. In this way, current from the active layer generated during the electrochemical reaction is transferred to the outside circuit directly through the conductive air diffusion layer, thus the metal collector is not needed. This two-layer air cathode without a current collecting substrate is simple but the electrical resistance is greater than the one with a current collecting substrate in the active layer.
Therefore, it is one object of the present invention to provide a novel air cathode, having a conductive air diffusion layer and a current collecting substrate not in the active layer. The cathode of the present invention is simple to make and has very low electrical resistance.
It is another object of the present invention to provide for metal air cells having high current densities, high working voltages and high power outputs, so that these metal air cells can meet the demands of high power consuming hearing aids.